There is a case where, when a listener wears headphones on the head of the listener to hear sound reproduced signals with both ears of the listener, audio signals reproduced by the headphones are normal audio signals that are provided to speakers located at the right and left in front of the listener. In such a case, it is known that a phenomenon so-called inside-the-head sound localization occurs in which a sound image reproduced by the headphones is trapped inside the head of the listener.
As techniques that solve this problem of the inside-the-head sound localization phenomenon, for example, Patent Literature 1 and Patent Literature 2 disclose a technique called virtual sound image localization. This virtual sound image localization causes headphones or the like to perform reproduction as if sound sources, for example, speakers are present at presupposed positions such as the right and left positions in front of a listener (to virtually localize the sound image at the positions).
In the case of multi-channels including three or more channels, as with a case of two channels, speakers are disposed at virtual sound image localization positions of the respective channels, and head-related transfer functions for the respective channels are measured by, for example, reproducing impulses. Then, the impulse responses of the head-related transfer functions obtained by the measurement may be convolved with audio signals to be provided to drivers for 2-channel sound reproduction of the right and left headphones.
Now, recently, multichannel surround sound systems such as 5.1 channel, 7.1 channel, and 9.1 channel, have been employed in sound reproduction or the like accompanying the reproduction of a video recorded in an optical disk. Also in the case where audio signals in this multichannel surround sound system are subjected to the sound reproduction by 2-channel headphones, the use of the above-described method of virtual sound image localization to perform sound image localization (virtual sound image localization) in conformity with each channel is proposed (e.g., Patent Literature 3).